Electron processing of semiconducting material



March 19, 1963 B. A. KULP ETAL 3,032,162

ELECTRON PROCESSING OF SEMICONDUCTING MATERIAL Filed Oct. 24. 1960 5 I? Q INTE RSTITIAL AFTER ATOM BEFORE CRYSTA L l6 I' MEV. ELECTRONS INVENTORS RAYMOND H. KELLEY BERNARD A. ULP

ATTORN Y United States Patent Force Filed Oct. 24, 1960, Scr. No. 64,634

2 Claims. (Cl. 204-154) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without the payment of any royalty thereon.

This invention relates to the processing of semiconducting material for controlling the interstitial atoms and the vacancies in the space lattice of the semiconducting material.

A background for understanding this invention as claimed is derived from publications such as Radiation Eifects in Solids by G. J. Dienes and G. H. Vineyard published in 1957 by Interscience Publishing Company, New York City,'New York; Crystal Structures by R. W. G. Wycotf, published in 1957 by Interscience; representative U.S. issued patents such as 2,911,533 to A. C. Damask; 2,588,254 to K. Lark-Horovitz et 211.; patents numbered 2,860,251; 2,842,466; 2,817,613; 2,787,564 etc.; Van Nostrands Encyclopedia published in 1958 by D. Van Nostrand Company, Inc., Princeton, New Jersey; and the publication Displacement of the Sulfur Atom in CdS by Electron Bombardment by B. A. Kulp and R. H. Kelley in the Journal of Applied Physics, volume 31, No. 6, pages 1057 to 1061, inclusive, published in June 1960, which last publication is a part of this disclosure.

This invention teaches the use of electrons, protons, neutrons and alpha particles as powerful tools for bombarding semi-conducting materials in successfully produc ing crystals with space lattices that have only vacancies without the accompanying interstitial atoms; materials with interstitial atoms without vacancies; and intermediate materials with controlled proportions of interstitial atoms and vacancies. The disclosed process is of special importance as applied to crystals grown from vapor phase deposition and particularly the sulfides of cadmium and zinc as well as elemental semiconductors such as silicon and germanium.

The object of this invention is to provide a control process for producing crystalline material of a predetermined space lattice composition of interstitial atoms and vacancies.

In the accompanying drawings:

FIG. 1 schematically illustrates a view down the c-axis of a cadmium sulfide crystal lattice containing an interstitial axis;

FIG. 2 schematically illustrates the impingement of an electron on an interstitial atom before and after collision;

FIG. 3 schematically illustrates the introduction of interstitial atoms into the lattice of a crystal;

FIG. 4 schematically illustrates the crystal with interstitial atoms being inserted within its lattice; and

FIG. 5 schematically illustrates the crystal in FIG. 4 with interstitial atoms at one level deep inside the crystal and vacancies of the host atoms at a second level deeply inside the crystal.

High energy electron bombardment of a solid results in the creation within the solid of vacancies and interstitial atoms. The numbers of vacancies and interstitial atoms varies linearly with the magnitude, time duration and area application of the electron flux.

The impingement of electrons on a material causes the diffusion of interstitial atoms through the lattice of the material by the transfer of the momentum of the electrons from the electrons to the interstitial atoms.

The processes that are disclosed herein remove interstitial atoms from the lattice of a crystal or add interstitial atoms to the lattice of the crystal in a predetermined conrolled degree or extent.

Interstitial atoms are, in efiect an impurity in the lattice of the crystal. The presence of interstitial atoms within a crystal lattice eflects both the electrical and the fluorescent or optical properties of the crystal, such as a particular semiconductor material or the like, much as does a chemical impurity in the same lattice.

Cadmium sulfide crystals irradiated with ultra violet light at the temperature of liquid nitrogen which is --\196 C. or 77 K., display a green fluorescence band that consists of a number of distinct lines about 80 angstrom units apart with maximum intensities at 5140, 5220 and 53.00. One angstrom unit is equal to 10- cm.

This green fluorescent band may be removed by the electron bombardment of crystals of cadmium sulfide of a thickness from .050 to .005 inch thick in the energy range of from 15 kv. upwardly to one million volts.

Conversely this green fluorescence may be produced in a crystal of cadmium sulfide by the difiusion of sulfur atoms into the lattice in interstitial positions under the 1 influence of electron bombardment in the same range of i energy. At the same low temperature of 196 C. cad- 1 mium sulfide crystals with cadmium interstitial atoms display fluorescence at 6,000 angstroms. This fluorescence is removed by the electron bombardment of thin crystals from .050 to .005 inch thick, in the energy range of from 30 kv. upwardly to one million volts.

Fluorescence at 6000 A. can be produced in cadmium sulfide crystals which do not previously fluoresce by the diffusion into the cadmium sulfide crystal lattice of interstitial cadmium atoms.

The process that is disclosed herein is based on the phenomenon of electron induced diffusion whereby a beam of electrons from an electron gun, a Cockcroft- Walton accelerator, a Van de Gratf accelerator or the like is caused to strike a thin target from .050 .to .015 inch thick of CdS or the like, whereupon the electrons collide with the atoms of cadmium and of sulfur that are in interstitial positions and drive them through the lattice. Where the crystal is sufliciently thin, such as in the lower part of the range from .050 to .005 inch thick, these atoms that are struck by electrons are driven out of the bottom of the crystal and are deposited on the foil or the like that supports the crystal.

No threshold energy for this process has been found down to 15 kev..for the sulfur interstitial atoms but electron energies greater than 27.5 kev. are required to move the cadmium interstitial atoms through the lattice.

Bombardment at room temperature of a cadmium sulfide crystal that is carried out above the threshold of kev. for the displacement of the interstitial sulfur atoms from the lattice, creates sulfur atom vacancies while both sulfur and cadmium interstitial atoms will be removed, leaving the cadmium sulfide material with only sulfur vacancies.

Bombardment at room temperature of a cadmium sulfide crystal and that is carried out above the threshold of about 350 kev. for the displacement from the cadmium sulfide crystal lattice of cadmium atoms, creates both cadmium and sulfur vacancies. The sulfur vacancies can be filled subsequently leaving only cadmium vacancies, or conversely the cadmium vacancies can be filled leaving only sulfur vacancies, as will appear hereinafter.

The production of a material with a controlled number of interstitial atoms is accomplished by coating the crystal material, such as cadmium sulfide with a predetermined quantity of the desired material, such as sulfur or cadmium and bombarding atoms of the coating material through the interface and into the cadmium sulfide lattice.

The placing of sulfur atoms in interstitial positions within the crystal lattice'of a cadmium sulfide crystal is accomplished'by depositing a little sulfur on top of the cadmium sulfide crystal, placing the crystal in a preferably evacuated furnace and raising the furnace temperature to melt the sulfur and spread it over the surface in a thin layer and then cool the crystal that then bears on one of its faces a thin coating of sulfur. Alpha beta and gamma sulfur melt in the range of from 95 to 120 C. and boil at 444.6 C.

If preferred, the crystal of cadmium sulfide without the addition of sulfur may be placed in an evacuated chamber with an amount of sulfur in a boat near the crystal. The boat temperature is increased to 445 C., under which conditions a layer of sulfur is evaporated out of the boat onto the cadmium sulfide crystal. The crystal then is removed from the chamber with the layer of sulfur on its surface.

The sulfur coated crystal produced by either method is subjected at a predetermined temperature to electron bombardment below the threshold for the production of new sulfur vacancies and illustratively in the direction perpendicular to the c-axis of the crystal. As disclosed by the inventors on page 1060 of the Journal of Applied Physics article, the sulfur coating illustratively is 6.8 micrograms per square centimeter thick and is bombarded with 100 kev. electrons at 100 C. for about 2.3 microampere-hours per square centimeter. The sulfur atoms from the sulfur laye rare driven into the cadmium sulfide crystal lattice.

After irradiation the crystal fiuoresces bright green at the temperature of liquid nitrogen at '196 C. 'under ultra violet stimulation where the sulfur has been deposited. The underside of the crystal glows green over a larger area of the crystal than the top.

This phenomenon supports the theory that the electron induced diffusion of the interstitial atoms actually occurs. The larger area of fluorescence on the bottom of the crystal than on the top supports the theory that both the electrons and the interstitial atoms spread out as they pass through the cadmium sulfide crystal lattice. A spectrogram of the green fluorescence showed three bands with maximum intensities at about 5140, 5225, and 5310 A.

In a corresponding process cadmium, silicon, germanium, or other material can be inserted into interstitial positions within a crystal lattice. It will be apparent that combinations of irradiations above the threshold for the displacement of illustratively either cadmium or sulfur atoms to create vacancies, followed by the bombardment of impurity atoms such as silver or copper can result in these impurities being deposited in lattice points.

The crystals illustratively are bombarded in a Cockcroft-Walton electron accelerator at room temperature in a Vacuum illustratively of 2X10- mm. Hg. Steady direct current electron currents of between 2 and 30 microamperes per square centimeter of crystal surface irradiated are used. Green emission is produced by bombardment at 130 kev. for 40 microampere-hours per square centimeter of crystal area irradiated. The green emission persists to a total of 160 microampere hours per square centimeter. Whisker crystals that are bombarded at 120 kev. for 240 na.-hr./cm. fiuoresced red at 196 C. under ultra violet stimulation. The crystals concerned illustratively are from .050 to .005 inch thick.

Cadmium sulfide exhibits photoconductivity at wavelengths shorter than the intrinsic band edge. When a cadmium sulfide crystal is simultaneously irradiated with band gap light of 5200 angstroms or shorter wavelength and with infrared light in the regions of 0.9 and 1.4 microns, the photoconductive current is less than that when the crystal is irradiated with band gap light alone. One micrn=l0 angstroms=3.937 10- inch. This phenomenon is called the quenching of photoconductivity by infrared radiation.

The capacity of a crystal to quench photoconductivity is considerably decreased when the crystal undergoes electron bombardment at energies in the vicinity of kv. If certain sulfide crystals of cadmium are irradiated with electrons of energy less than kv., which is the energy that is necessary to displace a sulfur atom from its lattice point, the electrical conductivity of the crystals decreases with the electron bombardment in an approximately logarithmic fashion until a minimum in the curve is reached.

In the Damask patent FIG. 1 is shown a graph of the electrical resistivity of alpha brass plotted against irradiation time in hours with an electron flux of 2.6 l0 electrons per square centimeter of irradiated area per second of about 2 mev. energy at a temperature of 50 C.

Lowering of a crystal electrical resistance at energies less than the displacement energy for a sulfur atom from the sulfide crystal space lattice is attributed to the removal of sulfur interstitial atoms. The sulfur interstitials are electron traps. The electrons released following the removal from a sulfide crystal of sulfur interstitial atoms go into the conduction band and contribute to the conductivity of the crystal.

Certain crystals of cadmium sulfide that have been in radiated with band gap light at -196 C. store electrons in their conduction bands, as explained in the application Serial Number 4581 filed January 25, 1960 for an Energy Storage Device by Donald C. Reynolds, Douglas M. Warschauer and Charles H. Blakewood. The ability of va crystal to store electrons in the conductivity band of a represents the sulfur atom in a lattice point and atom 2 represents the cadmium atom in a lattice point. The atom 3 is an atom in a so-called interstitial or non-lattice position.

In FIG. 2 of the accompanying drawings an electron 4 travelling in the direction 6 is represented as impinging on a stationary interstitial atom 5' and imparts the electrons kinetic energy to the atom in the direction 8 and with the electron deflected in the direction 7. In the event that the interstitial atom 5 exists on the outside face of a crystal when it is struck by the electron 4, then the interstitial atom 5 is driven out of the crystal lattice and is deposited on the material, such as aluminum, glass, steel or the like beneath the crystal.

FIG. 3 represents schematically the introduction of interstitial atoms into the lattice of a crystal by a uni directional electron beam 10 that drives atoms of a thin overlay 11 of a desired material such, for example, as sulfur, cadmium, copper, silver or the like, into the space lattice of a crystal 12 of cadmium sulfide or the like, as interstitial atoms in the lattice.

Illustratively, a crystal 12 of cadmium sulfide, which under electron bombardment at the temperature 196 C. does not display green fluorescence, may be coated with a thin layer 11 of elemental sulfur a few micrograms per square centimeter thick. The sulfur layer 11 is irradiated by the electron beam 10'of 100 kv. and of an intensity of a few microamperes per square centimeter within a temperature range of from 196 C. to 20 C. for about 30 minutes. This period and strength of irradiation introduces interstitial sulfur atoms into the cadmium sulfide crystal lattice.

The resulting crystal 12 displays the phenomenon of edge emission and displays electrical properties such as resistance, photo conductance and the like that were not possessed by the crystal prior to its irradiation, the irradiation effects being established only to the depth of penetration of the electrons, such as in a thin layer on the bombarded surface of the crystal. A crystal of a thickness penetrated by the electron beam has atoms of the sulfur layer 11 irradiated as interstitial atoms distributed throughout the crystal volume.

The irradiation of an originally N-type thin crystal creates a P-N junction therein by the presence of interstitial atoms that are electron traps and a thin layer of P-type material on the bombarded surface. The irradiation of an originally P-type material thin crystal by using the electron beam to drive electron donor interstitial atoms into the crystal lattice results in the creaation of an N-type layer on the bombarded surface of the originally P-type crystal. The thickness of the N- type layer in the parent crystal is controlled by controlling the energy of the electron beam. Thus, if 1,000,- 000 volt electrons are used, a crystal layer .050 inch thick is effected in cadmium sulfide while if a 100,000 volt electron energy is used a layer only about .005 inch thick is effected.

In FIGS. 4 and 5 of the accompanying drawings, a crystal 15 of cadmium sulfide is bombarded with one million electron volts electrons 16 froman accelerator until 10 electrons strike the crystal, then, as indicated in FIG. 5, an area 17 that is deep inside the crystal 15 contains an excess of interstitial atoms of the host crystal 15 and the area 18 contains an excess of vacancies of the host atoms.

For example, a crystal of cadmium sulfide bombarded as shown in FIG. 4 for 17 hours at 20 C. with 4 microamperes per square centimeter of bombarded area displayed intense green edge emission in the area 17 of FIG. 5, which area was 0.50 inch thick and occurred .050 inch inside the bombarded face of the crystal 15. Green fluorescent edge emission of cadmium sulfide crystal at the temperature 196 C. of liquid nitrogen results from the presence within the crystal of sulfur interstitial atoms.

This display of green fluorescene edge emission demonstrates that interstitial atoms that are created in the region 18 of the crystal 15 in FIG. 5 are driven by electron induced dififusion into the region 17. The electrical properties of resistance and conductance of the resultant crystal are very different in the region 18 and in the region 17 as compared with the same properties in the original crystal 15 and these changes are effected by controlling the number of vacancies in the region 18 and the number of interstitials within the region 17.

In this manner a desired configuration of NPN and of PNP devices is created. The control of the interstitial atom deposition in the crystal lattice of cadmium sufide crystals, silicon crystals and germanium crystals controls both the optical and the electrical properties of the crystals.

It is to be understood that the materials, the process steps, the time, the temperatures, the electrical and physical values and the like that are expressed herein are illustrative and that modifications may be made therein without departing from the spirit and the scope of the present invention.

We claim:

1. The process of introducing interstitial atoms of sulfur into the lattice of cadmium sulfide which comprises applying to a cadmium sulfide crystal surface a layer of 6.8 micrograms of sulfur per square centimeter crystal surface area, maintaining the sulfur coated cadmium sulfide crystal in the temperature range of from 196 to 20 C. during a 30 minute .period of electron bombardment, and bombarding the sulfur coated surface of the cadmium sulfide crystal with kev. electrons for 2.3 microampere hours per square centimeter of bombarded area, and thereby driving sulfur atoms from the sulfur layer on the surface of the crystal into the lattice of the cadmium sulfide crystal.

2. The process of introducing cadmium atoms into a cadmium sulfide crystal which comprises applying to the surface of the cadmium sulfide crystal a layer of about 7 micrograms of cadmium per square centimeter of crystal surface area, maintaining the cadmium coated crystal in the temperature range of from C. to 20 C. during a 30 minute period of electron bombardment, and bombarding the cadmium coated surface of the cadmium sulfide crystal with 100 kev. electrons for 2.3 microampere hours per square centimeter of bombarded area and thereby drive cadmium atoms from the cadmium layer on the surface of the crystal into interstitial positions within the crystal lattice.

References Cited in the file of this patent UNITED STATES PATENTS 2,563,503 Wallace Aug. 17, 1951 2,709,232 Thedieck May 24, 1955 2,750,541 Ohl June 12, 1956 2,860,251 Pakswer et al. 1- Nov. 11, 1958 2,945,793 Dugdale July 19, 1960 OTHER REFERENCES Davis et al.: Nucleon Bombarded Germanium Semiconductors, AEOD 2054, US. Atomic Energy Commission, June, 1948, pages 1-3. 

1. THE PROCESS OF INTRODUCING INTERSTITIAL ATOMS OF SULFUR INTO THE LATTICE OF CADMIUM SULFIDE WHICH COMPRISES APPLYING TO A CADMIUM SULFIDE CRYSTAL SURFACE A LAYER OF 6.8 MICROGRAMS OF SULFIDE PER SQUARE CENTIMETER CRYSTAL SURFACE AREA, MAINTAINING THE SULFUR COATED CADMIUM SULFIDE CRYSTAL IN THE TEMPERATURE RANGE OF FROM -- 196* TO 20* C. DURING A 30 MINUTE PERIOD OF ELECTRONS BOMBARDMENT, AND BOMBARDING THE SULFUR COATED SURFACE OF THE CADMIUM SULFIDE CRYSTAL WITH 100 KEV. ELECTRONS FOR 2.3 MICROAMPERE HOURS PER SQUARE CENTIMETER OF BOMBARDED AREA, AND THEREBY DRIVING SULFUR ATOMS FROM THE SULFUR LAYER ON THE SURFACE OF THE CRYSTAL INTO THE LATTICE OF THE CADMIUM SULFIDE CRYSTAL.
 2. THE PROCESS OF INTRODUCING CADMIUM ATOMS INTO A CADMIUM SULFIDE CRYSTAL WHICH COMPRISES APPLYING TO THE SURFACE OF THE CADMIUM SULFIDE CRYSTAL A LAYER OF ABOUT 7 MICROGRAMS OF CADMIUM PER SQUARE CENTIMETER OF CRYSTAL SURFACE AREA, MAINTAINING THE CADMIUM COATED CRYSTAL IN THE TEMPERATURE RANGE OF FROM -- 150* C. TO 20* C. DURING A 30 MINUTE PERIOD OF ELECTRON BOMBARDMENT, AND BOMBARDING THE CADMIUM COATED SURFACE OF THE CADMIUM SULFIDE CRYSTAL WITH 100 KEV. ELECTRONS FOR 2.3 MICROAMPERE HOURS PER SQUARE CENTIMETER OF BOMBARDED AREA AND THEREBY DRIVE CADMIUM ATOMS FROM THE CADMIUM LAYER ON THE SURFACE OF THE CRYSTAL INTO INTERSTITIAL POSITIONS WITHIN THE CRYSTAL LATTICE. 